Digital record discs for recording information signals such as digitized audio signals, video or image signals have become quite common, as have record disc reproducing apparatus for reproducing the recorded information data. Further, some digital record discs, e.g., so-called CDs (Compact Discs) have recently become used as ROM (Read Only Memory) devices for computers, so-called CD-ROMs.
On such a digital record disc, digital data intended for high density recording are recorded in the form of a spiral bit string on one side thereof and reproduced through reading the bit string by means of pickup means such as an optical pickup using laser beams. The optical pickup of the record disc reproducing apparatus follows the bit string by means of a tracking control system. CD players or video disc players are further used to process the read-out signal in a predetermined manner so as to convert the signal into a replica of the original analog form signal which was recorded, i.e., an audio or video signal, as an output.
As is well known, address information may also be recorded on the record disc which may be detected for a data search or a track jump operation, so that reproduction at a desired track location can be easily and quickly achieved. In particular, a selected address corresponding to the desired track location is preset, and the pickup is rapidly moved across or transverse to the disc tracks until the target track corresponding to the selected address is reached. During such a data search operation, the distance that the pickup should jump or transverse tracks is calculated by, for example, a microcomputer in accordance with the address of the track at which the pickup is presently located and the address of the target track. Then the pickup is moved that distance radially across the tracks of the disc by applying a pickup transfer drive voltage to a pickup transfer motor such as a linear motor. The pickup transfer drive voltage causes the pickup to move across or transverse tracks until the pickup transfer drive voltage ceases.
Conventionally, data reading devices such as CD-ROM drivers, CD players, etc., are provided with an information data search operation system to quickly read out a desired information signal recorded on a disc such as a CD or a CD-ROM. The search operation is realized by moving a pickup in the radial direction of the disc to the location of desired information to be read out. That is, the search operation is carried out by a so-called track jump movement in which the pickup jumps across or transverse a number of tracks.
Referring now to FIG. 1, a conventional data reading device will be described in brief. FIG. 1 is a block diagram showing the conventional data reading device for a disc player. In FIG. 1, a pickup 10 is provided for optically reading out data from a disc (not shown) such as a CD or a CD-ROM. The pickup 10 comprises an optical system such as a laser beam source, an objetive lens, a phototransducer, etc. and a mechanical system such as a lens support for movably supporting the objective lens in the pickup 10.
The pickup 10 outputs a signal responding to an optical signal applied from the disc to the pickup 10. The output signal is applied to a data processing circuit 12 and a tracking error detection circuit 14. The tracking error detection circuit 14 generates a tracking error singal S.sub.1. The tracking error singal S.sub.1 is obtained in response to a deviation of the pickup 10 from the center of a record track. The tracking error singal S.sub.1 is applied to a first fixed terminal 16a of a mode switch 16.
In the data reading mode, a movable terminal 16b of the mode switch 16 is connected to the first fixed terminal 16a under the control of a system controller 18 such as a microcomputor. Thus, the tracking error singal S.sub.1 is applied to an adder 20. The adder 20 is provided for introducing the tracking error singal S.sub.1 and a pickup transfer speed signal S.sub.2, which will be described later, into the same circuit. The tracking error singal S.sub.1 and the pickup transfer speed signal S.sub.2 are applied to both a pickup drive system 22 and an objective lens drive system (not shown). The objective lens drive system moves the objective lens in the direction across or transverse the tracks. Thus, a tracking servo for the pickup 10 is carried out.
The pickup drive system 22 comprises an integration compensation circuit 24, a drive amplifier 26 and a pickup transfer motor 28. The integration compensation circuit 24 compensates the tracking error singal S.sub.1 and the pickup transfer speed signal S.sub.2 so that some influences in the tracking error singal S.sub.1 and the pickup transfer speed signal S.sub.2 caused by nonlinearity of the mechanical system are reduced. The tracking error singal S.sub.1 and the pickup transfer speed signal S.sub.2 are applied to the pickup transfer motor 28 through the drive amplifier 26. Thus, the pickup transfer motor 22 gradually moves the pickup 10 in the direction toward the periphery of the disc to follow the spiral track of the disc.
The data reading device further comprises a pickup transfer speed detector 30 such as a frequency generator. The pickup transfer speed detector 30 is associated with the pickup 10 for generating the pickup transfer speed signal S.sub.2 responding to the transfer speed of the pickup 10. The pickup transfer speed signal S.sub.2 is introduced into the pickup drive system 22 through the adder 20. The pickup transfer speed signal S.sub.2 operates to dump the movement of the pickup transfer motor 28, in the servo control manner. Thus, the movement of the pickup 10 is stabilized.
Further, the data reading device comprises a track jump operation system 32 for carrying out the search operation. The track jump operation system 32 comprises the pickup transfer speed detector 30, an integrator 34, a target location indicating voltage generator 36, a comparator 38 and the system controller 18.
When an operation for searching a specific data to be read is commanded, the data reading device is changed from the data reading mode by the system controller 18.
In the search operation mode, the system controller 18 turns the movable terminal 16b of the mode switch 16 to the second fixed terminal 16c. Further the system controller 18 activates the integrator 34 so that the content of the integrator 34 is initialized. The system controller 18 further decides a transfer direction of the pickup 10 by judging whether the specific data to be read is located on a track position inner or outer than the present location of the pickup 10.
The integrator 34 is associated with the pickup transfer speed detector 30 for integrating the pickup transfer speed signal S.sub.2 applied from the pickup transfer speed detector 30. Thus, the integrator 34 outputs an integration voltage Vp. The integration voltage Vp output from the integrator 34 increases in response to the progress of movement of the pickup 10.
The system controller 18, e.g., the microcomputor calculates a distance between the present location of the pickup 10 and the target location of the specific data to be read. Then, the system controller 18 sets a target location indicating voltage Vt in the target location indicating voltage generator 36.
Both the integration voltage Vp output from the integrator 34 and the target location indicating voltage Vt generated from the target location indicating voltage generator 36 are applied to the comparator 38. The comparator 38 compares the integration voltage Vp with the target location indicating voltage Vt. Thus, an error voltage Ve corresponding to the difference between the voltages Vp and Vt is output from the comparator 38. The error voltage Ve is introduced into the pickup drive system 22 through the mode switch 16. The error voltage Ve drives the pickup transfer motor 28 so that the pickup 10 moves or jumps across the tracks toward the target location of the specific data to be read.
When the pickup 10 reaches the target location, the integration voltage Vp increases to the value almost the same as the target location indicating voltage Vt (Vp=Vt). Then, the error voltage Ve from the comparator 38 is eliminated to the zero voltage so that the movement of the pickup 10 finishes and the data search operation is completed. In response to the completion of the data search operation, the system controller 18 turns the data reading device to the data reading mode.
In the data search operation mode, the pickup transfer speed signal S.sub.2 is also introduced into the pickup drive system 22 through the adder 20. The pickup transfer speed signal S.sub.2 operates to dump the movement of the pickup transfer motor 28 in the negative feedback manner, in similar to the operation in the data reading mode. Thus, the pickup 10 is settled to the target track without overruning the target track. If the pickup transfer speed signal S.sub.2 is not introduced into the pickup drive system 22 in the data search operation, the pickup 10 is apt to overruning the target track. When such an overruning occurs, the pickup 10 approaches the target track through an oscillatory movement across the target track. Thus, the data search operation takes a long time period. Furthermore, if the target location of the specific data to be read is near to the periphery of the disc, the pickup 10 easily goes to the outside of the disc. When such overrun of the pickup 10 has occurred, the focus servo will be no longer carried out for the optical system in the pickup 10. As a result, the data reading operation becomes impossible or an enormous time will be required for restoration of the data reading operation.
The pickup transfer speed signal S.sub.2 introduced into the pickup drive system 22 prevents such an inconvenience. However, the conventional data reading device still has some drawbacks as described below.
The pickup transfer speed signal S.sub.2 is used for dumping the movement of the pickup 10 in the servo control manner. As a result, the pickup 10 can stablely follow the track in the data reading mode. In the data search mode, the pickup 10 can be settled on the target track without overruning the target track. However, the pickup 10 can not reach rapidly the target track. This is because a relatively large value of the pickup transfer speed signal S.sub.2 is always applied to the pickup drive system 22 during the data search operation, especially at the beginning of the data search operation. The large value of the pickup transfer speed signal S.sub.2 excessively dumps the movement of the pickup 10. Thus, the transfer speed of the pickup 10 toward the target track is reduced. This causes that the data search operation to take a long time.